1. Field of the Invention
The present invention relates to an information recording medium a reproducing apparatus and a recording apparatus, which reads out information from the information recording medium which moves, and particularly relates to an information recording medium to be recorded and/or reproduced through an optical device, and a reproducing apparatus and a recording apparatus for such an information recording medium.
2. Description of the Related Art
Until now, there is existed a system for reading out information from an information recording medium, which moves. In such a system, reproduction is performed by using a device such as an optical device, a magnetic device and a capacitive device. A system, which records and/or reproduces by an optical device, is used extensively in daily living. With respect to a read only information recording medium to be reproduced by using a light beam having a wavelength λ of 650 nm, for example, mediums such as a DVD (Digital Versatile Disc) Video prerecorded with video information, a DVD-ROM (Digital Versatile Disc-Read Only Memory) prerecorded with a program or like, and a DVD Audio disc and an SACD (Super Audio Compact Disc) prerecorded with musical information are well known.
Further, there is provided a DVD-R (Digital Versatile Disk-Recordable) as a recordable recording and reproducing type information recording medium, which uses dye. Furthermore, there is provided a DVD-RAM (Digital Versatile Disc-Random Access Memory), a DVD-RW (Digital Versatile Disk-ReWritable) and a DVD+RW (Digital Versatile Disk+ReWritable) as a recording and reproducing type information recording medium, which use phase change. Moreover, there is provided an ASMO (Advanced Storage Magneto-Optical) disc, an ID (intelligent image) disc and a GIGAMO (GIGA-byte Magneto-Optical) disc as a recording and reproducing type information recording medium using photo-magnetism.
On the other hand, in order to increase recording density of an information recording medium a study for shortening a wavelength of laser beam has been continued for a long period of time. A second harmonic generating element invented recently and a semiconductor light emitting element composed of gallium nitride system compound (disclosed in the Japanese Patent No. 2778405, for example), emit light having a wavelength λ of approximately 350 to 450 mm. Consequently, these elements can become an important light emitting element, which increases a recording density sharply.
In addition thereto, design for an objective lens coping with such a wavelength has been advanced, particularly, a lens having a numerical aperture (NA) of more than 0.7, which exceeds the NA 0.6 utilized for a DVD disc, is currently developed.
As mentioned above, a reproducing apparatus for an information recording medium having capability such that a wavelength λ is shortened to 350 to 450 mm and an NA is more than 0.7 has been developing. It is expected that an optical disc system, which has a higher recording density extremely exceeding that of a current DVD disc, can be developed by using these techniques.
By using such a light beam having a shorter wavelength than one for a DVD disc and a lens having a higher NA, an information recording medium having an extremely higher recording density can be realized. However, coma aberration also increases extremely when an information recording medium is slanted. Consequently, an information recording medium of which thickness for light transmission is made extremely thinner than that of a DVD disc is required. Actually, a disc system called a “DVR land groove” has been proposed. In the disc system, by using a light emitting element having a wavelength of 405 nm and an objective lens having an NA of 0.85, a thickness of disc for light transmission is designed for 0.1 mm.
With referring to FIGS. 1 and 2, a conventional information recording medium is explained.
FIG. 1 is a cross sectional view of a conventional information recording medium according to the prior art.
FIG. 2 is a fragmentary plan view, partially enlarged, of the conventional information recording medium shown in FIG. 1.
As shown in FIG. 1, an information recording medium 100 is composed of a recording layer 120 and a light transmission layer 110, which are laminated on a substrate 130 in order. The substrate 130 is formed with a microscopic pattern 200. The recording layer 120 is formed on the microscopic pattern 200 directly. The microscopic pattern 200 has microscopic patterns composed of land sections L1 and L2 (hereinafter generically referred to as “land section L”) and groove sections G1 through G3 (hereinafter generically referred to as “groove section G”).
While recording, as shown in FIG. 2, a record mark M is formed on both the land section L and the groove section G (it is called a land-groove recording method.)
With paying attention to dimensions of the microscopic pattern 200, with defining that a minimum distance between the centers of adjacent groove sections G is a pitch P (a minimum distance between the centers of adjacent land sections L is also the pitch P), the land section L and the groove section G are formed so as to satisfy a relation P>S with respect to a reproduction spot diameter S of a light beam.
Further, the reproduction spot diameter S can be calculated by an equation S=λ/NA, where λ is a wavelength of a laser beam for reproduction and NA is a numerical aperture of an objective lens. In other words, the pitch P is designed in order to satisfy a relation P>λ/NA.
In the information recording medium 100, an information recorded in the recording layer 120 is read out by irradiating a reproducing light beam incident on the light transmission layer 110. The information is taken out through the light transmission layer 110 after the reproducing light beam has been reflected by the surface of the recording layer 130 and reproduced.
The inventors of the present invention performed an experiment for recording and reproducing the information recording medium 100 actually manufactured by using a light emitting element radiating a light beam having a single wavelength within a range of 350 to 450 nm and an objective lens having a higher NA of 0.75 to 0.9, and then it is found that a cross erase phenomenon was remarkable.
The cross erase phenomenon is a phenomenon such that an information to be recorded in a land section L, for example, is recorded in a groove section G with overlapping a signal previously recorded in the groove section G when recording the information in the land section L. In other words, it is such a phenomenon that an information previously recorded in a groove section G is erased by recording another information in a land section L.
Further, this phenomenon can also be noticeable in a reverse case. That is, the cross erase phenomenon is also recognized if a previously recorded information in a land section L is observed when recording an information in a groove section G.
If the cross erase phenomenon occurs, as mentioned above, an information recorded in an adjacent track is damaged. In a case of an information system having larger capacity, an amount of lost information becomes excessively large. Consequently, affection to a user is enormous. Therefore, it is considered for such an information recording medium 100 that an information shall be recorded only in either land section L or groove section G. However, recording capacity of an information recording medium will decrease and a merit of the information recording medium having a potential of recording in higher density will decline.